Removing iron from magnesium alloys



Patented Dec. 8, 1942 REMOVING mom mom MAGNESIUM ALLOYS Graydon E. Holdeman, Midland, Mich, assignor to The Dow Chemical Company, Midland, Mich., a corporation of Michigan No Drawing. Application December is, 1941, SeriaLNo. 423,183

4 Claims.

This invention relates to the removal of iron impurities from magnesium base alloys.

For some time it has been appreciated that the tendency of uncoated articles of magnesiumbase alloys to corrode on exposure to moist air and to aqueous solutions is in someway dependent upon the presence of traces of iron in the metal. More recently, it has been shown that there is a certain minute but critical proportion of iron, e. g., about 0.002 per cent in the case of magnesium-base alloys containing aluminum, be-

low which these metals are highly corrosion-resistant, but above which they are relatively corrodible. It is evident, then,-that, in order to render iron-contaminated magnesium-base al loys resistant to attack, the iron content thereof must be reduced to below the critical corrosion tolerance limit. Unfortunately, however, since at least part of the iron is in actual physical solution in the metal when molten, many of the removal-processes described in the art are ineffective in reducing the iron concentration to a sufficiently low value.

It is accordingly an object of the present invention to provide a simple and convenient meth od of treating iron-contaminated magnesiumbase alloys to reduce the concentration of iron therein to a proportion well below the critical corrosion tolerance limit, i. e., to below about 0.002 per cent for most alloys.

The invention depends upon the discovery that iron is virtually insoluble, i. e., soluble to an extent well below 0.002 per cent by weight, in molten magnesium-base alloys containing a small proportion of beryllium. Thus, when beryllium is incorporated into the molten, iron-contaminated alloy, the dissolved iron is rendered insoluble and precipitates, until less than 0.002 per cent remains in solution. The resulting alloy, since it containsiron in a proportion below the critical corrosion limit, is highly corrosionresistant.

In removing dissolved iron impurities from iron-contaminated magnesium-base alloys according to the invention, then, the metal to be purified is first melted, ordinarily under a protective saline flux. Beryllium is then incorporated into the molten alloy in a proportion suflicient to precipitate the iron impurities, as by agitating the melt with-metallic beryllium, usually in the form of a finely-divided beryllium-containing alloy, e. g., an aluminum-beryllium alloy. Alternatively, a small proportion of a reducible beryllium compound, conveniently anhydrous berylmagnesium-base alloy, the compound being reduced to form metallic beryllium, which becomes incorporated into the melt. Following incorporation of the beryllium, whereby the iron impurities are rendered insoluble and precipitate, the molten alloy is separated from the precipitated iron impurities, usually by maintaining the melt quiescent for a time sufllcient to allow the insoluble impurities to settle out of suspension, and then decanting or ladling off the supernatant substantially iron-free alloy.

The initial proportion of beryllium to be incorporated into the molten magnesium-base alloy to effect precipitation of the iron impurities varies slightly for diflerent alloys, but is in general at least about 0.005 per cent by weight; a part of this beryllium may settle out of the melt during the iron-settling operation, even if no present. With an initial beryllium concentration in the metal of about 0.005 per cent, the

iron impurities settle out of suspension usually within a few hours; higher initial concentrations of beryllium, approaching the maximum which can be incorporated in the molten alloy, viz., 0.01 to 0.02 per cent, decrease the iron-settling time to a comparatively short period, often 0.2 to 1.0 hour. in general, the presence of sufficient beryllium is insured by agitating the melt with beryllium in a substantial excess of the requisite 0.005 per cent. Usually 0.1 to 0.2 per cent by weight of beryllium is added as beryllium chloride, or, in the treatment of aluminum-containing magnesium alloys, as an aluminum-beryllium alloy containing 1 to 30 per cent of beryllium; with such excess, only a small part of the added beryllium actually becomes incorporated into the metal, probably by actual physical solution therein; the remainder of the beryllium is merely in suspension, and settles out the melt when agitation is discontinued.

The temperature of the molten alloy during the beryllium treatment is conveniently well above the melting point of the metal. In general, temperatures of' about 1600 to 1700 F. permit more rapid iron-settling, although temperatures as low as 1350 F. or even less are operable.

At the lower temperatures, the concentration of beryllium remaining in the metal after iron removal is lower than at the higher temperatures.

If desired, the metal may be superheated at temperatures above 1600 F. to effect grain refinelium chloride, may be stirred into the molten 5 vention:

ganese, the balance being commercial magnesium, and containing at least 0.030 per cent iron impurity, was melted under a protective saline flux and heated to a temperature of 1500 F. Beryllium in aproportion of 0.04 per cent by weight of the alloy was then added with agitation in the form of an aluminum-beryllium alloy containing than strictly 'limitative, the invention being coextensive in scope with the following claims.

1. The method of removing dissolved iron impurities from molten iron-contaminated magnesium-base alloys which comprises incorporat per cent beryllium, the melt being heated to a temperature of 1700 F. to insure complete dis-' maintained quiescent and allowed to cool to a temperature of 1400 F. during a period of 50 minutes, during which time the iron impurities and most of the added beryllium settled out of suspersion of the beryllium. The "melt was then pension. The supernatant metal was ladled into iron-free molds and allowed to solidify. 0n analysis,.the metal was found to contain 0.0009 per cent iron and 0.0031 per cent beryllium.

Example 2' in a proportion of 0.2 per cent by weight was added 6 in the form of an aluminum-beryllium'alloy containing 10 per cent of beryllium. The resulting melt was then stirred vigorously for 0.5 hour, after which it was held quiescent at 1700 F. for a few minutes to allow the iron impurities and most of the added beryllium to settle out of suspension. The resulting supernatant melt was then poured into molds and allowed to solidify. On analysis, the metal was found to contain 0.0002 per cent iron and somewhat over 0.01 ,per cent beryllium.

It should be explained that the precise analytical determination of minute proportions of beryllium in magnesium-alloys is diflicult, and that the numerical values of beryllium concentration set forth above are therefore only the best approximations available. All'beryllium concentrations herein reported were determined by painstaking application of standard spectrochemical procedures:

The foregoing description is illustrative rather mg beryllium into the molten metal in a proportion suflicient to precipitate the iron impurities, and separating the precipitated impurities from the molten metal.

2. The method of removing dissolved iron impurities from molten iron-contaminated magnesium-base alloys which comprises agitating the' molten metal with a sufllcient quantity of a substance selected from the class consisting of beryllium andreducible beryllium compounds to incorporate therein at least about 0.005 per cent by weight of berrylium, whereby th iron impurities are rendered insoluble and precipitate; and separating the molten metal from the precipitated iron impurities. T

3. The method of removing dissolved iron impurities from molten iron-contaminated magnesium-base alloys which comprises agitating the molten metal with a suflioient quantity of a substance selected from the class consisting of beryllium and reducible beryllium compounds to incorporate therein at least about 0.005 per cent by weight of beryllium, whereby the iron impuritiesare rendered insoluble and precipitate; discontinuing agltation and maintaining the melt quiescent for a time sufllcient to permit the insoluble iron impurities to settle out of suspension; and separating the supernatant iron free metal from the settled impurities.

4. The method of treating an iron-contami nated aluminum-containing magnesium-base ailoy to remove the iron therefrom to a value below 0.002 per cent which comprises agitating the molten metal with a small proportion of an aluminum-beryllium alloy in a quantity suflicient to incorporate into the melt at least about 0.005 per cent beryllium, whereby the iron impurities are rendered insoluble and precipitate; discontinuing agitation and maintaining the melt quiescent for a time suflicient to permit the insoluble iron impurities to settle out of suspension; and separating the supernatant iron-free metal from the settled impurities. M g A GRAYDON E. HOLDEMAN. 

